Connecting multiple units (hard drives; monitors etc.) in series, often with FireWire or Thunderbolt.
Daisy chaining is the method of connecting multiple units, such as hard drives or monitors, in series. Some connection types support this better than others; for instance, FireWire supports daisy-chained hard drives quite well while USB does not. Also, one of the benefits of Thunderbolt is the amount of functionality it supports in this area — according to Apple, you can even daisy chain up to two high definition Thunderbolt Displays with it.